1. Field of the Invention
The present invention relates to a spinel-type lithium titanium oxide (Li4Ti5O12)/reduced graphite oxide (graphene) composite and a method of preparing the same, and more particularly, to a method of preparing a spinel lithium titanium oxide/graphene composite having excellent electrochemical properties capable of simplifying a manufacturing process and shortening a manufacturing time using microwave associated solvothermal reaction and post heat treatment.
2. Discussion of Related Art
Graphene is a carbon structure composed of a two-dimensional (2-D) nanosheet single layer in which sp2 carbon atoms are formed into a hexagonal honeycomb lattice. Since graphene was separated from graphite using a peeling method developed by the Geim research staff (Great Britain) in 2004, reports on graphene have continued to be published. Graphene has come into the spotlight as a leading new material because it has a very high specific surface area (a theoretical value of 2600 m2/g) with respect to its volume and shows excellent electronic conductivity and physical and chemical stabilities (a typical value of 8×105 S/cm in an aspect of quantum mechanics).
In particular, graphene serves as an effective template on which a nano-sized transition metal oxide can be deposited due to its high specific surface area, excellent electric conductivity and physical and chemical stabilities. When a nanocomplex is formed with a transition metal, graphene may be used in an unlimited variety of applications such as energy storage materials (a lithium ion secondary battery, a hydrogen storage fuel cell, an electrode of a supercapacitor, etc.), gas sensors, medical engineering microparts, and a highly functional composite in a variety of devices.
However, graphene is not easily peeled even when it is in a solution phase because of the van der Waals's interaction between graphene layers due to the presence of sp2 carbon bonds on a surface of the graphene. Also, graphene is not mainly present as single layer graphene but thick multilayer graphene, and readily re-stacks when it is peeled off. Therefore, when a complex material with a transition metal oxide is synthesized in a solution phase using graphene as a precursor, it is difficult to use a high specific surface area of single layer graphene and to form a uniform complex structure, which serves as a factor preventing the use of the transition metal oxide.
On the other hand, graphite oxide is a material in which a number of oxygen functional groups are introduced into a surface of a graphite layer having a graphite-layered structure obtained by subjecting graphite to strong oxidation. Therefore, graphite oxide may be used as a precursor when graphene is mass-produced using a method such as chemical reduction or thermal peeling. Unlike graphene, graphite oxide may be easily dispersed into single layer graphite oxide or graphene oxide due to numerous oxygen functional groups on a surface of the graphite oxide when the graphite oxide is coated with another solution including a water system and is subjected to ultrasonic treatment. Therefore, when a complex material with a transition metal oxide is synthesized using graphene oxide uniformly dispersed in a solution phase as a precursor, graphene oxide may serve as a template on which a nano-sized transition metal oxide can be uniformly deposited. However, since the various oxygen functional groups introduced into the surface of the graphene oxide through an oxidation process are generated by partial breakup of sp2 bonds of graphene, the electric conductivity may be degraded. Therefore, when a complex with a nano-sized transition metal oxide is formed using graphene oxide, in order to use the excellent electric conductivity of graphene, a subsequent process of removing the oxygen functional groups from the surface of the graphene oxide and recovering the sp2 bonds of the graphene using a reducing agent or a hot treatment process is necessarily required after formation of a complex material with the nano-sized transition metal oxide.
In recent years, Li4Ti5O12 having a spinel structure has come into the spotlight as an anode material for a lithium ion battery. This is because the anode material is hardly changed in volume during charging/discharging cycles, which allows the lithium ion battery to have a long stable lifespan property (cycling) and avoid reduction of electrolytes in an electrode surface. However, conventional Li4Ti5O12 having a spinel structure is difficult to manufacture on a nanosized scale due to its limits in manufacturing processes, and shows poor capacity and rate capability as the lithium battery anode material because of its poor conductivity. In addition, since a large amount of time (for example, 24 hours) is required to synthesize Li4Ti5O12, many problems should be solved in advance for it to be applied to the lithium secondary battery. Accordingly, ways and means to solve the above-mentioned problems are still required.